timer_ptr2id and timer_id2ptr are used to convert between
application-visible timer_t and struct timer_node *. timer_ptr2id was made
to use void * instead of timer_t in 49b650430e ('Update.') for no reason.
It happens that on Linux timer_t is void *, so both that change and this
commit are no-ops there, but not on systems where timer_t is not void *.
Using timer_ptr2id for filling sival_ptr also does not make sense since that
actually is a void *.
* sysdeps/pthread/posix-timer.h (timer_ptr2id): Cast to timer_t
instead of void *.
* sysdeps/pthread/timer_create.c (timer_create): Do not use
timer_ptr2id to cast struct timer_node * to void *.
